Abstract
Deep VI CCD photometry of ∼4200 stars in a 3'.9×3'.9 central field of NGC 3109, a bright Magellanic-type spiral galaxy (Sm IV), has been obtained. The tip of the red giant branch (TRGB) of low-mass stars is found to be at I TRGB =21.55±0.1 mag and (V−I) TRGB =1.7±0.2 mag. The mean metallicity of the red giant branch (RGB) stars has been measured using the color of the RGB stars at M I =−3.5 mag, (V−I) −3.5 =1.41±0.07 mag: [Fe/H]=−1.6±0.2 dex. The distance modulus to NGC 3109 has been estimated using the I magnitude of the TRGB to be (m−M) 0 =25.45±0.15 mag for an adopted foreground extinction of A I =0.08 mag, showing a very good agreement with the Cepheid distance modulus estimate based on very recent CCD photometry, (m−M) 0 =25.5±0.2 mag (Capaccioli, Piotto, & Bresolin)
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